Program Fundamentals

/* HelloWorld.java* The classic “Hello, world!” program*/

class HelloWorld {public static void main (String[ ] args) {System.out.println(“Hello, world!”);

}}

/* HelloWorld.java … <etc.> */

• /* … */ and // …• These are comments• Everything between /* and */ or after // is

ignored by the compiler• They explain the program and its parts to

humans– You, me, the TA, your teammate, your

colleagues, and anyone else that might want to understand your program

class HelloWorld {• “class” is a java keyword

– keywords are words with special meaning in a programming language

• A class is a named collection of – data objects, and– operations on those data objects

• Note how this matches our design!– This is object oriented programming!

• The braces { } surround the things in the class

public static void main (String[] args) {

• main() is a java method– A method is a named set of operations within

a class– The parentheses ( ) follow the name of the

method– The braces surround the body of the method– Program vs. applets– Every program has a main( ) function

• That’s where the program starts executing

System.out.println(“Hello, world!”);

• This is the body of the main( ) method• This is the instructions that run when

main( ) is executed• This code prints something on the screen

– Whatever is between the quotes “ ”

Compiling and Running

• Compiler translates human-readable code into machine-readable code

• The name of the .java file usually matches the name of the class it contains

• Java bytecode is machine independent– machine code, binaries, and executables are not

HelloWorld.java(source code)

HelloWorld.class(bytecode)Java Compiler

Lexical Elements

• Composed of characters• The lowest-level components of a

program– White space– Comments– Keywords– Identifiers– Literals– Operators– Punctuation

Thrown out by the compiler

Converted into tokensby the compiler

White space

• Space, tab, and newline• Separate tokens not otherwise separated

by punctuation• Make the code readable• Can’t appear in a keyword, identifier, or

literal• Otherwise ignored by the compiler

Comments

• Provide additional information to a person reading the code

• Separates tokens like white space• Single-line comment: // …• Multi-line comment: /* … */• Ignored by the compiler• Important part of any good program!

Keywords (aka Reserved words)

• Special words that can’t be used for anything else: abstract, boolean, byte, case, catch, char, class, const, continue, default, do, double, else, extends, final, finally, float, for, goto, if, implements, import, instanceof, int, interface, long, native, new, package, private, protected, public, return, short, static, super, switch, synchronized, this, throw, throws, transient, try, void, volatile, while

• null, true, false – predefined like literals

Identifiers

• Names for different elements of a java program: classes, methods, and variables

• Defined by the programmer• Any sequence of letters and digits starting with a

letter (including $ and _)– Except Java keywords and null, true, and false

• Examples– Ok: HelloWorld, println, data, first_name, a7, java– Not ok: 123, x+y, int, data?, first name

Literals

• Constants – primitive program elements with a fixed value

• Five types of constants in java– int – 1, 79, -23, 0– double – 1.5, 2.7, 3.14159, -0.3– boolean – true, false– char – ‘a’, ‘A’, ‘z’, ‘2’, ‘3’, ‘$’– String – “Hello”, “foo”, “123”, “123(*&T^%”

Operators and Punctuation

• Operators specify an action to take on data– +, -, *, /, %, ++, --, etc.– Really just shorthand for specific methods on that

data• Punctuation separates or encloses program

elements or parts– ; , ( ) { } .

• Type, Precedence, and Associativity• By the way: ., !, *, #, $, &, ^, @, ~, |, /, ->

Data Types and Variable Declarations

• Every data object has an associated type that specifies– What it is– What operations it supports

• Primitive types– Numeric: byte, short, int, long, float, double –

numbers in different sizes and formats– Character: char - characters– Logical: boolean – true, or false– Can be created using literals or as the result of

operations (17, 2+3, etc.)

Data Types and Variable Declarations (cont.)

• Class types– String, Button, Point, etc.– Composed of other class types and primitive

types– Created with the class keyword– Over 1500 classes in standard Java

Variables

• Data objects– Have a specified type– Have a value of that type

• Variable declaration<type> <identifier>;<type> <identifier1>, <identifier2>,

<identifiern>;

Variable Initialization

• Examplesint age;boolean flag1;double hang_time; // C style identifierString firstname;Button clickToExit; // Java style identifierint first, second, third;

// HelloWorld2.java - simple variable declarations

class HelloWorld2 {public static void main(String[ ] args) {String word1, word2, sentence;

word1 = “Hello, ”;word2 = “world!”;sentence = word1.concat(word2);System.out.println(sentence);

}}

strings vs Strings vs. Identifiers vs. Variables

• string – a particular data value that a program can manipulate

• String – a Java type - data objects of this type can contain strings

• Variable – a data object, has an identifier, a type, and a value

• Identifier – the name of a particular class, variable, or method

• Example: String animal = “elephant”;

// StringVsId.java – contrast Strings & Identifiers

class StringVsId {public static void main(String[ ] args) {String hello = “Hello, world!”;String stringVary;stringVary = hello;System.out.println(stringVary);stringVary = “hello”;System.out.println(stringVary);

}}

User Input

• Most interesting programs get input from the user

• Lots of ways to do this• For now we will use tio (terminal I/O)

– If this doesn’t work, seehttp://www.cse.ucsc.edu/~charlie/java/tio/doc/install.html

// SimpleInput.java-read numbers from the keyboardimport tio.*; // use the package tio

class SimpleInput {public static void main (String[] args) {int width, height, area;System.out.println("type two integers for" +" the width and height of a box");width = Console.in.readInt();height = Console.in.readInt();area = width * height;System.out.print("The area is ");System.out.println(area);

}}

Calling Predefined Methods• A method is a named group of

instructions– We’ve seen main( ), System.out.println( ),

• We execute a method by calling it– We call a method by putting its name in the

program where we want it to be executed• Method names don’t have to be unique

– Identified by the object name -System.out.println( )

• function is another name for method

Passing Parameters to Methods

• Many methods take inputs: parameters• Parameters are passed to the method by

placing them between the parentheses• Example: System.out.println(“Hello”);

– “Hello” is the parameter passed to System.out.println( )

• Multiple parameters are separated by commas

print( ) and println( )

• System.out.print( ) and System.out.println( ) print out strings and the primitive types

• Difference: println( ) puts a newline at the end

• Explicit newline is represented by ‘\n’, as in System.out.print(“Hi\nScott\n”);– Same as System.out.println(“Hi”);

More on print( ) and println( )

• Concatenation with ‘+’– ‘+’ allows multiple things in a print( ) statement– System.out.print(“The value is: ” + value);

• Be careful with numeric types– Given int a = 5, b = 7;– System.out.println(“The value is: ” + a + b); prints out

“The value is: 57”– System.out.println(“The value is: ” + (a+b)); prints out

“The value is 12”– System.out.println(a + b); prints out “12”

Number Types

• Two basic representations for numbers– Integer: whole numbers– Floating point: fractional numbers and very

big numbers• Bit

– The smallest element of storage in a computer

– Can be either 0 or 1– Bigger numbers are stored as a sequence of

bits

Representing Numbers with Bits

• A sequence of bits is interpreted as a binary number– 00, 01, 10, 11 binary = 0,1,2,3 in decimal– Read Appendix A

• A byte is 8 bits– Smallest addressable unit in a computer– Can contain any number between –128 and

127

Integer Types

-9223372036854775808

to 9223372036854775807

64long

-2147483648 to 2147483647

32int

0 to 6553616char

-32768 to 3276716short

-128 to 1278byte

Range of ValuesNumber of BitsType

Floating point types

15 decimal digits

+/-10-324 to +/-10+308

64double

7 decimal digits+/-10-45 to +/-10+38

32float

Approximate Precision

Approximate Range of Values

Number of bits

Type

Char

• char is a special integer type– Holds numeric values that represent

Unicode characters– Examples:

• Special characters• ‘\\’, ‘\b’, ‘\r’, ‘\”’, ‘\f’, ‘\t’, ‘\n’, ‘\’’, ‘’

434238574948676665999897

‘+’‘*’‘&’‘9’‘1’‘0’‘C’‘B’‘A’‘c’‘b’‘a’

Numeric Literals

• Integer literals– Default to type int– Can be specified as long by ending with ‘L’– 24, 1003, 123887699888L– Octal: begin with ‘0’, as in 0217– Hex: begin with “0x” as in 0xB3D

• Floating point literals– Default to type double– Can be specified as float with ‘F’– 3.7, 2.9, 3.1416F, 1358494.34792098

Numbers vs. Chars vs. Strings• The number 49 is different from the char 49 is

different from the string “49”• int or literal 49 = 32 bit binary number

00000000000000000000000000110001 binary• char 49 = 16 bit binary number and represents

the Unicode character ‘1’0000000000110001 binary

• String “49” = ‘4’ + ‘9’ + 0 = 52 57 0 = 00000000010100100000000001010111 0000000000000000 binary

Arithmetic Expressions

• Operators:– Addition: +– Subtraction: -– Multiplication: *– Division: /– Modulus (remainder): %

• Types: char, byte, short, int, long, float, double

Rules of Mixed-Mode Arithmetic

1. An arithmetic operation on objects of the same type yields a result of that type

2. An arithmetic operation on objects of different types first promotes smaller types to larger type– Any operand is a double ⇒ promoted to double– Otherwise, any float ⇒ promoted to float– Otherwise, any long ⇒ promoted to long– Otherwise, any int ⇒ promoted to int– And then rule 1 applies

Details

• Any result value that is too big for the result type will be undefined– Solution: force promotion when necessary by

using a variable or literal of the larger type or by casting one operand to a suitably larger type

– Example: (float)5, (long)a, 5.0, 4f• Integer types storing floating point values

will have the fractional part truncated– towards 0

// MakeChange.java - change in dimes and penniesimport tio.*; // use the package tio

class MakeChange {public static void main (String[] args) {int price, change, dimes, pennies;System.out.println("type price (0:100):");price = Console.in.readInt();change = 100 - price; //how much changedimes = change / 10; //number of dimespennies = change % 10; //number of penniesSystem.out.print("The change is : ");System.out.println(dimes + " dimes, " + pennies + “ pennies");

}}

Type Conversion

• Implicit (in mixed-mode arithmetic)• Explicit (casting)• Widening

– From “smaller” to “larger type”– All information is retained

• Narrowing– From “larger” to “smaller” type– Information may be lost– Result may be meaningless

• Common mistake: int z = 3.0/4.0;

Assignment Operators

• <variable> = <rightHandSide>;• What happens?

– Right hand side is evaluated– The result is placed in the variable

<variable>• Examples:

a = 0;a = b + c;a = Console.in.readInt( );a = b = c;

More Assignment Operators

• = , += , -= , *= , /=, %= , >>= , <<= , &= , ^= , |=

• += is pronounced “plus equals”, etc.• All but ‘=‘ are shorthand• Example:

a += b; is shorthand for a = a + b;• The others work the same way

Increment and Decrement Operators

• ++ is shorthand for “add one to the variable”i++; and ++i; are shorthand for i = i + 1;

• -- is shorthand for “subtract one from the variable”i--; and --i; are shorthand for i = i - 1;

• Location determines order of evaluationint a, b=0;a = ++b; // result: a = 1 and b = 1;a = b++; // result: a = 0 and b = 1;

Order of Evaluation

• In expressions with multiple operators, order matters!

• Example:j = (3 * 4) + 5; // result: j = 17j = 3 * (4 + 5); // result: j = 27

Precedence and Associativity• Precedence specifies which operators are

evaluated first• Associativity specifies order when operators

have equal precedence• Parentheses ( ) override these

– They force whatever is inside to be evaluated as a unit

• Example:x = 3 + 4 * 5; // result: x = 23x = (3 + 4) * 5; // result: x = 35

• Look at Appendix B for details

Programming Style

• Comments– At the top of every file– At the top of every class definition– At the top of every method definition– At the top of every non-trivial block of

instructions• Identifiers should be short and

meaningful• Readability, understandabilty, clarity,

elegance

Java Naming Conventions

1. Class names start with uppercase and embedded words are capitalized, e.g. HelloWorld

2. Methods and variables start with lowercase and embedded words are capitalized, e.g. readInt, data, toString, loopIndex

3. $ should not be used and _ marks you as an old C programmer